Antifreeze proportion question?

I have flushed out my antifreeze system and having some strange results. I wanted to change from dexcool to normal silicate based coolant so i flushed the system five times and finally got a clear stream to come out. I then proceeded to use a gallon of coolant mixed with a gallon of distilled water. I just checked it with a hygrometer and I am only achieving around a -10 degree Fahrenheit lower protection. How would i remedy this problem? Anything over 60 is not advised by the manufacturer